Methods and implantable medical systems that implement exposure modes of therapy that allow for continued operation during exposure to a magnetic disturbance

ABSTRACT

Implantable medical systems enter an exposure mode of operation, either manually via a down linked programming instruction or by automatic detection by the implantable system of exposure to a magnetic disturbance. A controller then determines the appropriate exposure mode by considering various pieces of information including the device type including whether the device has defibrillation capability, pre-exposure mode of therapy including which chambers have been paced, and pre-exposure cardiac activity that is either intrinsic or paced rates. Additional considerations may include determining whether a sensed rate during the exposure mode is physiologic or artificially produced by the magnetic disturbance. When the sensed rate is physiologic, then the controller uses the sensed rate to trigger pacing and otherwise uses asynchronous pacing at a fixed rate.

TECHNICAL FIELD

Embodiments relate to implantable medical devices that provide modes oftherapy such as providing cardiac pacing. More specifically, embodimentsrelate to implantable medical systems that allow for an exposure mode oftherapy to be controlled to allow for continued operation duringexposure to a magnetic disturbance like that created by a magneticresonance imaging (MRI) scan or other medical procedures.

BACKGROUND

Implantable medical devices may perform various functions in order todeliver modes of therapy to a patient. For example, cardiac stimulationdevices like pacemakers and defibrillators may sense electricalphysiologic signals in some modes of therapy in addition to providingelectrical pacing signals to one or more chambers of the heart. Somemodes of therapy that sense a physiologic signal then use that signalwhen determining how to control the pacing signal.

Patients that have an implantable medical device may be exposed tomagnetic disturbances like those caused by MRI scans or other medicalprocedures. These magnetic disturbances may result in the device sensingsignals that are not actually physiologic but are artificially createdby the magnetic disturbances. If the device is allowed to control thepacing signal based on the sensed artificial signal, then the pacingsignal may be inappropriate or even harmful for the patient. Therefore,it is commonplace to utilize an exposure mode of therapy during suchdisturbances where the exposure mode deactivates sensing or otherwiseignores the sensed signal and paces asynchronously in a pre-definedpacing configuration with a pre-defined pacing rate. This may not be anoptimal exposure mode of therapy, especially for certain patients havingparticular device types such as those with pacing ability but beingprimarily for defibrillation purposes. Likewise, the pre-defined pacingrate may not be an optimal rate of pacing for patients with other pacingneeds.

SUMMARY

Embodiments address issues such as these and others by applying variousconsiderations in order to determine an appropriate mode of therapyand/or an appropriate rate when the device is in an exposure mode suchas during times when a magnetic disturbance is present. For instance,the type of device may be considered to determine whether a pace capabledevice is primarily for pacing only or for defibrillation or for cardiacresynchronization with defibrillation abilities. A measure of the priorrate of cardiac activity, such as an average of a prior intrinsic rateor of a prior pacing rate, may be factored into the determination of anappropriate asynchronous rate. Additionally, the device may continue tosense the intrinsic rate and then analyze that intrinsic rate todetermine whether the intrinsic rate is physiologic or artificial. Thepacing mode may then be set to use a physiologic intrinsic rate totrigger the pacing signal or to use the asynchronous pacing rate andignore an artificial intrinsic rate.

Embodiments provide methods and devices controlling an exposure mode oftherapy of an implantable medical device. A device type of theimplantable medical device is determined and upon detecting a need toswitch to the exposure mode, the exposure mode of therapy of theimplantable medical device is selected based on the device type and apre-exposure mode of therapy. The exposure mode of therapy isimplemented at the implantable medical device by performing a firstsensing of a heart rate and when the first sensing of heart rateindicates at least a first amount of noise is present, implementing anasynchronous pacing mode and then making adjustments to at least onesensing setting prior to a next sensing of the heart rate and when thefirst sensing of heart rate indicates that at least the first amount ofnoise is not present, then implementing a triggered pacing mode.

Embodiments provide methods and devices controlling an exposure mode oftherapy of an implantable medical device. A type of an implantablemedical device is determined by determining whether the implantablemedical device is capable of providing a therapy to multiple chambers ofa heart. When the implantable medical device is not a type capable ofproviding therapy to multiple chambers of the heart, then it isdetermined if the device is of a type capable of defibrillation therapy.When the device is not capable of defibrillation therapy then a pacingpercentage of the device is compared to a first threshold. When thedevice is capable of defibrillation therapy then the pacing percentageof the device to a second threshold. The exposure mode is selected basedon device type and the pre-exposure mode by implementing a pacing modethat paces a same chamber of the heart as the pre-exposure mode when thepacing percentage exceeds the first threshold when the device is notcapable of defibrillation therapy or the second threshold when thedevice is capable of defibrillation therapy.

Embodiments provide methods and devices for controlling an exposure modeof therapy of an implantable medical device by determining if the deviceis a defibrillator type and if so, then determining if a lower pacingrate is lower than a rate threshold and if a ventricular pacingpercentage is lower than a pacing percentage threshold. If the lowerpacing rate is lower than the rate threshold and the pacing percentageis lower than the pacing percentage threshold, then choosing anon-pacing mode as the exposure mode of therapy. If the lower rate isnot lower than the rate threshold and/or the pacing percentage is notlower than the pacing percentage, then choosing a permanent pacing modeas the exposure mode of therapy.

Embodiments provide methods and devices for controlling an exposure modeof therapy of an implantable medical device by determining if a heartrate exceeds a first threshold. If not, then a pacing rate for theexposure mode is set to the heart rate plus a first adjustment. If so,then it is determined if the heart rate exceeds a second threshold thatis higher than the first and if a ventricular pacing percentage exceedsa pacing percentage threshold. If the heart rate does not exceed thesecond threshold and/or the ventricular pacing percentage does notexceed the pacing percentage threshold, then a pacing rate for theexposure mode is set to the heart rate plus a second adjustment butlimited to a specified maximum. If the heart rate does exceed the secondthreshold, the ventricular pacing percentage does exceed the pacingpercentage threshold, and the device type is other than aresynchronization therapy device, then the pacing rate is set to a fixedamount that is higher than the first threshold and lower than the secondthreshold and a pacing mode is set to VOO. If the heart rate does exceedthe second threshold, the ventricular pacing percentage does exceed thepacing percentage threshold, and the device type is a resynchronizationtherapy device, then the pacing rate is set to the heart rate plus thesecond adjustment but limited to the specified maximum.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the techniques as described in detailwithin the accompanying drawings and description below. Further detailsof one or more examples are set forth in the accompanying drawings andthe description below. Other features, objects, and advantages will beapparent from the description and drawings, and from the statementsprovided below.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of an implantable medical system that mayoperate according to various embodiments disclosed herein.

FIG. 2 shows an example of components of the implantable medical system.

FIG. 3 shows an example of inputs, considerations, and resulting outputdecisions when implementing an exposure mode of operation at theimplantable medical system.

FIG. 4 shows an example of logical operations that may be performed bythe implantable medical system to consider device type and prior rate ofcardiac activity when establishing the exposure mode of therapy.

FIG. 5A shows a first example of logical operations that may beperformed by the implantable medical system to consider whether a sensedintrinsic rate is physiologic or artificial and whether to utilizetriggered pacing or asynchronous pacing as the exposure mode of therapy.

FIG. 5B shows a second example of logical operations that may beperformed by the implantable medical system to consider whether a sensedintrinsic rate is physiologic or artificial and whether to utilizetriggered pacing or asynchronous pacing as the exposure mode of therapy.

FIG. 6 shows another example of inputs, considerations, and resultingoutput decisions when implementing an exposure mode of operation at theimplantable medical system.

FIG. 7 shows another example of logical operations that may be performedby the implantable medical system to consider device type based onwhether the device is pace capable and whether there is an atrialfibrillation in progress and also considers prior rates of cardiacactivity when establishing the exposure mode of therapy.

FIG. 8 shows another example of logical operations that may be performedby the implantable medical system to consider device type based on anumber of chambers being paced and considers prior rates of cardiacactivity when establishing the exposure mode of therapy.

FIG. 9A shows a first portion of another example of logical operationsthat may be performed by the implantable medical system to considerdevice type and consider prior rates of cardiac activity whenestablishing an asynchronous rate in preparation for considering anintrinsic rate to trigger pacing when establishing the exposure mode oftherapy.

FIG. 9B shows one example of a second portion of the logical operationsbeing continued from FIG. 9A where the intrinsic rate is considered todetermine whether to use triggered pacing.

FIG. 9C shows a second example of a second portion of the logicaloperations being continued from FIG. 9A where both atrial andventricular rates are considered to determine whether to use atrialand/or ventricular triggered pacing.

FIG. 9D shows a third example of a second portion of the logicaloperations being continued from FIG. 9A where an electrogram parameteris considered to determine whether to use triggered pacing.

FIG. 9E shows a fourth example of a second portion of the logicaloperations being continued from FIG. 9A where both atrial andventricular electrogram parameters are considered to determine whetherto use atrial and/or ventricular triggered pacing.

FIG. 10 shows another example of logical operations that may beperformed by the implantable medical system to consider device type andprior rates of cardiac activity when establishing the exposure mode oftherapy.

DETAILED DESCRIPTION

Embodiments provide an implantable medical system that employs variousconsiderations to determine an appropriate exposure mode of therapy suchas during times when the system is exposed to a magnetic disturbance.Considerations may include the type of implantable medical device thatis present in the system and the manner of use of the device inpre-exposure modes. Considerations may also include whether a sensedintrinsic signal is physiologic and therefore reliable for triggeringpacing or artificial and therefore unreliable. Considerations mayfurther include pre-exposure mode rates.

FIG. 1 shows an example of an implantable medical system according toembodiments disclosed herein. A patient receives an implantable medicalsystem 100 which, in this example, implantable medical system 100includes an implantable medical device 102 that has the ability toperform electrical sensing and pacing. The implantable medical device(IMD) 102 may be of various types and some of those types may offeradditional functionality such as defibrillation and/or cardiacresynchronization therapy. The IMD 102 may even be primarily fornon-pacing functions like defibrillation and/or cardiacresynchronization although may include the ability to pace if needed. Acollection 108 of electrical components is included to provide thesefunctions.

The implantable medical system 100 in some cases may also include one ormore electrical leads 104. The electrical lead(s) 104 are electricallyconnected to the IMD 102 via proximal contacts 110 on the leads 104 andelectrical connectors 112 of the IMD 102. The leads 104 includeelectrodes 106 on a distal end that interface with the body tissue tocapture electrical physiologic signals or deliver electrical pacingsignals.

The implantable medical system 100 in some cases may omit the use ofelectrical leads 104. In this embodiment, electrodes 106 integrated intothe IMD 102 that interface with the body tissue to capture electricalphysiologic signals or deliver electrical pacing signals.

Additionally, in some cases the implantable medical system 100 mayinclude external devices 114 such as hand-held controllers that arecapable of communicating wirelessly with the IMD 102. The wirelesscommunications 116 may be near field, arm's length, far field and thelike as is known in the art. The external device 114 may generatecommands to the IMD 102 to request information about the IMD 102 and/orto instruct the IMD 102 to operate in a particular way. In particular,in some cases the external device 114 may be used to manually switch theIMD 102 to enter an exposure mode of therapy. Alternatively oradditionally, the IMD 102 may have the ability to automatically detectmagnetic disturbances and then automatically enter an exposure mode oftherapy.

FIG. 2 shows an example of components of an IMD 102. The IMD 102 mayinclude a housing 202 that contains the various components. The IMD 102includes a controller 204 that may control the operations of the IMD 102by communicating with other components. The controller 204 may be ofvarious forms such as a general purpose programmable processor, adedicated purpose processor, hardwired digital logic, and the like. Thecontroller may also include internal or external memory havingcomputer-readable instructions that, when executed by controller 204cause controller 204 to perform various operations attributed to it inthis disclosure. The memory may include any volatile, non-volatile,magnetic, optical, or electrical media, such as a random access memory(RAM), read-only memory (ROM), non-volatile RAM (NVRAM), staticnon-volatile RAM (SRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or any other non-transitory computer-readablestorage media.

The IMD 102 may include telemetry 206 to communicate wireless withexternal devices such as the external device 114. As discussed above,the IMD 102 may communicate via one or more types of wirelesscommunications including near field, arm's length, far field and thelike. For instance, the telemetry 206 may include inductive coupling fornear field or arm's length, may include radio frequency far fieldfunctions such as those operating in the Medical Implant CommunicationService (MICS) band.

The IMD 102 also includes a therapy device 208 that may include one ormore engines for providing various therapy functions. For instance, thetherapy device 208 may include a configurable pacing engine 212 that maypace one or more chambers of the heart via corresponding one or moreelectrodes of the lead 106. The therapy device 208 may include aconfigurable sensing engine 214 to sense from one or more chambers ofthe heart. Likewise, depending upon the type of IMD 102, the therapydevice 208 may include a defibrillation engine 216 capable of providinghigh voltage defibrillation shocks. Again depending upon the type of IMD102, the therapy device 208 may include a cardiac resynchronizationengine 218 capable of providing cardiac resynchronization signals. Insome embodiments a single “engine” may be used for multiple types oftherapy, such as a single pacing engine that provides bradycardiapacing, anti-tachycardia pacing and/or cardiac resynchronization pacingtherapy.

The engine(s) may be implanted in the form of one or more modules. Forinstance, a therapy delivery module may provide the pacing,resynchronization, or/or defibrillation functions. In one example, anengine may include a low voltage (LV) therapy module for delivering lowvoltage pacing pulses using an extra-cardiovascular pacing electrodevector selected from various electrodes. LV therapy module may beconfigured to deliver low voltage pacing pulses, e.g., 8 V or less or 10V or less. One or more capacitors included in the LV therapy module arecharged to a voltage according to a programmed pacing pulse amplitude bya LV charging circuit, which may include a state machine. The LVcharging circuit may charge the capacitors to a multiple of the voltageof a battery included in a power source without requiring a transformer.At an appropriate time, the LV therapy module couples the capacitor(s)to a pacing electrode vector to deliver a pacing pulse to the heart.

An engine may additionally or alternatively include a high voltage (HV)therapy module that includes one or more high voltage capacitors. When ashockable rhythm is detected, the HV capacitor(s) is(are) charged to ashock voltage amplitude by a HV charging circuit according to theprogrammed shock energy. The HV charging circuit may include atransformer and be a processor-controlled charging circuit that iscontrolled by a control module. The control module applies a signal totrigger discharge of the HV capacitor(s) upon detecting a feedbacksignal from therapy delivery module that the HV capacitors have reachedthe shock voltage amplitude required to deliver the programmed shockenergy. In this way, control module controls operation of the highvoltage therapy module to deliver CV/DF shocks using defibrillationelectrodes and/or a device housing.

HV therapy module may be used to deliver cardiac pacing pulses. In thiscase, the HV capacitor(s) is(are) charged to a much lower voltage thanthat used for delivering shock therapies but may be higher than themaximum available pulse voltage amplitude produced by the LV therapymodule. For example, the HV capacitor may be charged to 40 V or less, 30V or less, or 20 V or less for producing extra-cardiovascular pacingpulses.

Compared to pacing pulses delivered by LV therapy module, pulsesdelivered by HV therapy module may have a higher voltage amplitude andrelatively longer pulse width for delivering higher energy pacing pulsesfor capturing the heart. More current may be delivered using a lowimpedance pacing electrode vector. Longer pulse width is attainable dueto a higher capacitance (and consequently higher RC time constant) ofthe HV capacitor(s). The LV therapy module may be capable of producing amaximum pulse voltage amplitude of up to and including 10 V. The maximumsingle-pulse pacing pulse width produced by LV therapy module may be 2ms. In some examples, LV therapy module may be configured to producecomposite pacing pulses comprising two or more individual pulses fusedin time to deliver a cumulative composite pacing pulse energy thatcaptures the heart. Techniques for delivering composite pacing pulsesare generally disclosed in the U.S. patent application Ser. No.15/367,516 and in provisional U.S. Pat. Application No. 62/262,412 andcorresponding pending U.S. patent application Ser. No. 15/368,197, allof which are incorporated herein by reference in their entirety. Themaximum composite pacing pulse width may be up to 8 ms or higher.

Additionally, the IMD 102 may include one or more sensors 210 fordetecting magnetic disturbances. For instance, Hall effect sensors maybe used to detect that a magnetic field of a particular intensity ispresent. This allows the controller 204 to then enter an exposure mode.The controller 204 may then distinguish whether the magnetic field isrepresentative of a programming trigger caused by a programming magnetbeing placed near the site of implantation of the IMD 102 or is amagnetic disturbance like that from an MRI machine or other source thatnecessitates an exposure mode of therapy. For instance, the controller204 may utilize various techniques to distinguish the programming magnetfrom an MRI machine. Examples include determining magnetic fieldstrength particularly at multiple locations, determining force resultingfrom the magnetic field, determining torque resulting from the magneticfield, determining magnetic field direction particularly as measured atmultiple locations, and the like to then determine if threshold are metthat are indicative of a magnetic field from an MRI machine. Examples ofsuch techniques are disclosed in U.S. application Ser. Nos. 13/046,158;13/456,891; 13/587,368; and 14/340,893, all of which are incorporated byreference herein in their entirety.

FIG. 3 shows a basic flow of information that may be used by acontroller, such as a controller within the external device 114 or theinternal controller 204 of the IMD 102, to make a determination as tothe appropriate exposure mode and rate of therapy that should beemployed. This basic flow of information may be used to invoke theappropriate exposure mode once it has been detected, either by manualprogramming or by automatic detection, that the IMD 102 should switch tothe exposure mode. The controller may gather information to be used asinputs 302. Examples of the inputs include various pieces of informationfrom the pre-exposure mode operation such as: pacing percentage for bothventricular and atrial pacing; adaptive and/or general cardiacresynchronization therapy programming including which ventricles arebeing stimulated; the average atrial and/or ventricular rate of pacing;the type of device including whether the device is an implantablecardiac defibrillator (ICD), an implantable pulse generator (IPG)primarily for pacing, a cardiac resynchronization therapy (CRT) devicewith pacing ability but not defibrillation, or a CRT device that alsoincludes defibrillation ability; and pacing mode programming includingwhich chambers are being paced.

The controller may then process this input information 302 viaconsiderations 304. For instance, the controller may determine whetherpacing is even needed during the period of time that exposure mode isactive, such as during an MRI scan. Factors include what the priorpacing percentage is and in the case of CRT, whether the cardiacresynchronization stimulation is only being applied to the leftventricle or to other chambers.

The considerations 304 may also include determining what the pacing modeshould be if pacing is needed, such as whether to pace the atrium,ventricle, or both. This may be based on the pre-exposure programming aswell as current status such as whether atrial fibrillation is occurringand whether a mode switch due to atrial fibrillation is warranted toavoid wasting atrial pacing signals.

The considerations 304 may also include determining the pacing rate. Forusing asynchronous mode of pacing, the fixed rate may be based on apre-exposure mode factor such as the average ventricular rate.Furthermore, consideration may be given to a pre-defined range of rates.For instance, for a given patient, it may be desirable to contain thefixed rate to somewhere between 85 and 100 paces per minute. Asdiscussed below in reference to FIG. 5A, in some embodiments it may bedesirable to also consider whether it is possible to sense an intrinsicrate that is physiologic rather than artificial while in the exposuremode and in that case trigger pacing from the intrinsic rate rather thanusing asynchronous pacing at the previously established asynchronousrate. The result of these considerations 304 is to produce outputs 306necessary to implement the particular exposure pacing mode rate.

FIG. 4 shows an example 400 of logical operations that a controller mayemploy to ultimately select the pacing mode and rate for the exposuremode of therapy. Initially, the type of IMD 102 is determined at a query404 that accesses the device type information 402. For instance, wherethe controller performing these operations is the external device 114being used to manually program the IMD 102 to enter the exposure mode,the external device 114 may send a wireless request for information atthe query 404 to obtain the device type 402 from the IMD 102. Where thecontroller 202 of the IMD 102 is the controller performing theseoperations either in response to a command from the external device 114to enter the exposure mode or where the IMD 102 has detected themagnetic disturbance and has automatically entered the exposure mode,then the controller 202 accesses the locally stored device typeinformation 402.

Regardless of which device performs these operations, query operation404 determines from the device type information 402 whether the IMD 102,and specifically the therapy device 208, is of the type that can providedefibrillation. For instance, query operation 404 may specifically checkfor an ICD or a CRT device with defibrillation. Operational flow thenproceeds to a query operation 408 when the IMD 102 does havedefibrillation ability or proceeds to query operation 418 when the IMD102 does not have defibrillation ability.

It is also to be appreciated that an external device 114 may implementthe query 404 and then provide the IMD 102 with instruction regardingwhether the controller 204 should then proceed with the remainder ofthese operations by beginning at the query operation 408 or at the queryoperation 418. The controller 204 may then proceed with the remainder ofthese operations where the controller 204 then utilizes pre-exposuremode information without being required to uplink that information tothe external device 114. However, where the external device 114 performsall of these operations or where the query 404 is based to some degreeon pre-exposure mode programming (e.g., CRT-D with defibrillationdeactivated to behave as a CRT-P), then the pre-exposure modeinformation is uplinked from the IMD 102 to the external device 114 andthe external device 114 downlinks the instruction for the properexposure mode and rate.

It is also to be noted that at query operation 404, it is possible insome embodiments that no pre-exposure mode information has been neededbecause the type of device is entirely hardware based and not affectedby pre-exposure programming. In that case an IMB 102 that has theability to automatically enter the exposure mode may be programmed bythe manufacturer regarding which branch of logic to use in FIG. 4 thatstems from the query operation 404. In that case, the initialprogramming being loaded onto the IMB 102 from the manufacturer utilizesthe device type information 402 to specify in the initial programmingwhich query operation should be the first operation to be performed bythe IMB 102 upon automatically entering exposure mode, either queryoperation 408 or query operation 418. In that case, the IMD 102 need notperform the query operation 404 upon entering the exposure mode sincethat query 404 was already resolved at the time of initial programming.

In any of these alternatives above, operational flow proceeds to query408 where the device type is such that defibrillation is available andproceeds to the query operation 418 otherwise. For device types leadingto the query operation 408, the controller determines from pacingpercentage information 406 stored by the IMD 102 whether the pacingpercentage (considering either atrial or ventricular) is greater than athreshold. The pacing percentage information 406 may be based on pacingactivity for some pre-exposure period immediately preceding the exposuremode, such as 15 minutes or for the entire day such as since midnight.For example, query 408 may determine whether the IMD 102 has been pacingthe ventricle more than 5% of the pacing cycles for the 15 minutespreceding the exposure mode. It will be appreciated that other pacingpercentage thresholds may also be applied but 5% is believed to beeffective for most patients. If the pacing percentage is lower than thethreshold, this indicates that it is appropriate for the patient to gowithout pacing for a short time such as for the duration of an MRI scan,and in that case the controller selects that there be no pacing viapacing mode OXO for the exposure mode as indicated at pacing state 414.While pacing state 414 shows the pacing mode as OOO, it will beappreciated that this is just one example of the OXO modes that may beused.

If the pacing percentage is at the query operation 408 is greater thanthe pacing threshold, then the controller determines at a queryoperation 412 whether the device is performing adaptive and/or generalCRT with left ventricle pacing only by referencing the pre-exposure modeinformation 410. If CRT is being used with only left ventricle pacing,then even though the pacing percentage is above the threshold, it isstill appropriate to stop the left ventricle pacing for a short periodof time and the controller selects that there be no pacing for theexposure mode as pacing mode OXO as indicated at pacing state 414.

If the controller finds that CRT is being used and more than merely leftventricle pacing is occurring, then the controller concludes that someform of pacing is needed. Therefore, the controller then proceeds to thequery operation 418 just as if query operation 404 had not found thatthe device was defibrillation capable (i.e., that the IMD 102 is a typethat is primarily for pacing instead of primarily for defibrillation).

At the sequence of query operations 418 and 422, the controller thendetermines what the pre-exposure pacing mode has been by referencing thepre-exposure mode pacing mode information 416. This pre-exposure pacingmode, also referred to as the permanent pacing mode, is AXX/AXXR,VXX/VXXR, or DXX/DXXR. The controller sets the exposure mode to pace thesame set of chambers as the pre-exposure mode, so where the pre-exposurepacing mode is AXX/AXXR, then in this embodiment the controller selectsthat there be atrial pacing for the exposure mode by using asynchronouspacing mode AOO as indicated at pacing state 420. Where the pre-exposurepacing mode is VXX/VXXR, then in this embodiment the controller selectsthat there be ventricular pacing for the exposure mode by usingasynchronous pacing mode VOO as indicated at pacing state 424. Where thepre-exposure pacing mode is DXX/DXXR, then in this embodiment thecontroller selects that there be dual chamber pacing for the exposuremode by using asynchronous pacing mode DOO as indicated at pacing state426.

Once the pacing mode for the exposure mode has been set, the controllerthen proceeds to a query operation 430 to determine an appropriateexposure mode pacing rate for the chosen pacing mode. The queryoperation 430 determines whether a pre-exposure mode rate of cardiacactivity as specified in pre-exposure mode information 428 exceeds arate threshold. The pre-exposure mode information 428 may be one ofvarious different pieces of information. For example, the pre-exposuremode information 428 may specify an average ventricular rate taken overa 15 minute period immediately preceding the exposure mode. Thispre-exposure mode average rate may be an average intrinsic rate that hasbeen sensed or it may be an average paced rate.

It will be appreciated that the rate threshold applied at the queryoperation 430 may vary but it has been found that 70 beats per minute isan effective example. If the threshold is not exceeded by thepre-exposure mode pacing rate, then the controller sets the asynchronouspacing rate to a pre-defined fixed value such as 85 beats per minute asindicated at the pacing rate state 432. It will be appreciated that thispre-defined fixed value may vary, but 85 beats per minute are known tobe acceptable for most patients. If the threshold is exceeded by thepre-exposure mode rate, then the controller sets the asynchronous pacingrate to a fixed value that is equal to the pre-exposure mode rate plus afixed adjustment but with an upper limit at the pacing rate state 434.For example, the controller may set that asynchronous pacing rate to thepre-exposure rate plus 15 additional paces per minute with an upperlimit of 100 paces per minute. The fixed adjustment may vary but it hasbeen found that an adjustment of 15 additional paces per minuteadequately avoids a competitive pacing condition. Furthermore, the upperlimit may vary but it has been found that an upper limit of 100 pacesper minute is adequate. Therefore, in this particular example, thepacing rate for the exposure mode will be set within the range of 85-100paces per minute.

FIG. 5A shows an example 500 of logical operations that may be used bythe controller 204 to continuously adapt the exposure mode to use atriggered pacing mode when possible and to otherwise use an asynchronousfixed pacing rate. In this example, the average rate, either intrinsicor paced, over some period of time preceding the exposure mode isobtained at the rate operation 502. When needed, the exposure mode, alsoreferred to as the Mill mode, is entered at the operation 504. Thecontroller then begins monitoring the sensing for an intrinsic rate at aquery operation 506.

Because a magnetic disturbance may result in sensing an artificialintrinsic rate that is not appropriate for triggering pacing, thecontroller 204 utilizes logic at the query operation 506 to determinewhether the sensed rate is a normal rate, a faster than normal rate, ora noise reversion which occurs where the rate may be within the normalrange but an abrupt change within the range is artificial. Thecontroller compares the sensed rate to at least one reference value, asdiscussed in more detail below, to make the determination.

When the sensed rate is determined to be normal by the controller, thenthe controller sets the exposure mode to triggered pacing of thechambers that were being paced in the pre-exposure mode as indicated atthe triggered pacing state 508. Thus, the triggered pacing mode may beAAT, VVT, or DDT. The normal rate may be confirmed by using a range ofrates known to be normal in general as the reference values to compareagainst the sensed rate. Alternatively, the reference values to compareto the sensed rate may be more specific to this particular patient byestablishing an upper limit for the range of normal rates by utilizingthe average intrinsic or paced pre-exposure rate plus an adjustment. Asanother alternative, the normal rate may be established by maintaining astatistical distribution, such as a histogram, of historicalpacing/sensing data and choosing the normal range to be those within aparticular range of percentiles. Such a histogram or other statisticaldistribution may instead be used to determine the adjustment to theaverage pre-exposure rate by choosing a rate in a larger percentile asthe upper limit of the normal range. For example, the upper limit to thenormal rate may be the average pre-exposure rate plus 50 beats perminute.

Where the controller detects that the sensed rate falls outside of thenormal range, then the controller sets the exposure mode to asynchronousfixed rate pacing as indicated at the pacing state 510. This ensuresthat the sensed rate, which is possibly artificial, does not produce aninappropriate pacing rate.

Where the controller detects that the sensed rate indicates a noisereversion, then the controller sets the exposure mode to asynchronousfixed rate pacing as indicated at the pacing state 520. The controllermay detect the noise reversion by comparing a current sensed rate toimmediately prior sensed rates collected during a set period of timethat precedes the sensing of the current intrinsic rate and looking forabrupt changes to the sensed rate that are artificial rather thanphysiologic. This may be done by comparing a difference in the currentand prior sensed rates to a threshold where the threshold is chosen todistinguish a physiologic change in rate from an artificial one. Forexample, a near instantaneous change of 50 beats per minute is morelikely to be artificial than physiologic.

Thus, using this example of FIG. 5A, the IMD 102 may enter an exposuremode and may transition as needed from triggered pacing to asynchronouspacing. It will be appreciated that the example of FIG. 5A may be usedin conjunction with the example of FIG. 4 to also control the exposuremode based on device type. For instance, if the IMD 102 is primarily fordefibrillation or CRT with defibrillation and the pacing percentage islow, then pacing may be avoided altogether, but when the example of FIG.4 determines that pacing should be done while in exposure mode, then theexample of FIG. 5A may be further employed to transition betweentriggered pacing and asynchronous pacing based on the results ofanalyzing the sensed rate.

FIG. 5B shows a second example 501 of logical operations that is similarto FIG. 5A with the operations 502, 504, 506, 508, and 510 being thesame as described above for FIG. 5A. However, when the query operation506 detects that the sensed intrinsic rate is higher than the normalphysiologic range and/or senses a high noise floor, asynchronous mode ischosen at asynchronous state 510 which leads to a query operation 512.Here, it is detected whether a revision counter that is incremented asdiscussed below now exceeds a revision counter threshold. This thresholdmay be set to various amounts depending upon preference for how manyattempts are desired to try to enter or re-enter triggered pacing modeupon noise being found. If the threshold is exceeded, then operationalflow returns to the query operation 506 to continue sensing with thecurrent sensing parameters. If the threshold is not exceeded, thenoperational flow proceeds to an operation 514.

At the operation 514, the sensing parameters may be adjusted so that thesensed signal may be reconsidered using the adjusted parameters in orderto potentially eliminate the noise. Eliminating the noise allows thephysiologic portion of the sensed signal to be revealed and if withinthe physiologic range then used to successfully perform triggered pacingvia the triggered pacing state 508. Examples of adjusting the sensingparameters include adjusting the sensitivity of the sensing in terms ofchanging the noise floor threshold. Another example includes adjustingfilter cut-off frequencies, bandwidth, and gain settings. Anotherexample includes applying active or dynamic noise cancellation. Once theadjustment is made, the revision counter may be incremented at anoperation 516 and then operational flow proceeds to the query operation506 to continue sensing using the new sensing parameters. If the sensedresult is within the normal physiologic range, then the triggered pacingmode is used via the triggered pacing state 508. Otherwise, asynchronouspacing mode continues via the asynchronous pacing state 510.

FIG. 6 shows another example of a basic flow of information that may beused by a controller, such as the external device 114 or the internalcontroller 204, to make a determination as to the appropriate exposuremode and rate of therapy that should be employed. This basic flow ofinformation may also be used to invoke the appropriate exposure modeonce it has been detected, either by manual programming or by automaticdetection, that the IMD 102 should switch to the exposure mode. Thecontroller may gather information to be used as inputs 602. Examples ofthe inputs include various pieces of information from the pre-exposuremode operation in addition to those shown in FIG. 3, such as: rateflashbacks and rate histograms, designations of whether the device is asingle chamber or dual/triple chamber device, as well as patientspecific information including age, implant indications, andcomorbidities.

The controller may then process this input information 602 viaconsiderations 604. As discussed for FIG. 3, the controller maydetermine whether pacing is even needed during the period of time thatexposure mode is active, such as during an MRI scan. Factors includewhat the recent average pacing percentage, right ventricle versus leftventricle pacing percentage, and implant indications.

The considerations 604 may also include determining what the pacing modeshould be if pacing is needed, such as whether to pace the atrium,ventricle, or both. This may again be based on the pre-exposureprogramming as well as current status such as whether atrialfibrillation is occurring and whether a mode switch due to atrialfibrillation is warranted to avoid wasting atrial pacing signals.

The considerations 604 may also include determining the pacing rate. Forusing asynchronous mode of pacing, the fixed rate may be based on apre-exposure mode factor such as the recent average atrial orventricular rate. Furthermore, consideration may be given to apre-defined range of rates as well as patient age, implant indications,and comorbidities. As with FIG. 3, it may be desirable to contain thefixed rate to a range but in this example, the range is specified asbeing between 70 and 110 paces per minute. Like that discussed above inFIG. 5A and as further discussed below in reference to FIGS. 9A-9C, insome embodiments it may be desirable to also consider whether it ispossible to sense an intrinsic rate that is physiologic rather thanartificial while in the exposure mode and in that case trigger pacingfrom the intrinsic rate rather than using asynchronous pacing at thefixed rate. The result of these considerations 604 is to produce outputs606 necessary to implement the particular exposure pacing mode rate.

FIG. 7 shows an example 700 of logical operations that may be used todetermine the exposure pacing mode to be implanted based on device type,where the device type considerations are whether the device is capableof pacing. The example 700 may further be used to determine whetherthere is some episode occurring at the time of entering the exposuremode that requires pacing of a given type or lack thereof. Initially,the type of IMD 102 is determined at a query 704 that accesses thedevice type information 702. For instance, where the controllerperforming these operations is the external device 114 being used tomanually program the IMD 102 to enter the exposure mode, the externaldevice 114 may send a wireless request for information at the query 704to obtain the device type 702 from the IMD 102. Where the controller 202of the IMD 102 is the controller performing these operations either inresponse to a command from the external device 114 to enter the exposuremode or where the IMD 102 has detected the magnetic disturbance and hasautomatically entered the exposure mode, then the controller 202accesses the locally stored device type information 702.

Regardless of which device performs these operations, query operation704 determines from the device type information 702 whether the IMD 102,and specifically the therapy device 208, is of the type that can providepacing either as the sole mode of therapy or as a mode that is inaddition to other modes such as defibrillation or cardiacresynchronization. Operational flow then proceeds to a query operation708 when the IMD 102 does have pacing ability or otherwise proceeds toset the exposure mode as a non-pacing mode as shown at the pacing modestate 710. While pacing mode state 710 shows the pacing mode as OOO, itwill be appreciated that this is just one example of the OXO modes thatmay be used.

As with the example in FIG. 4, is also to be appreciated that anexternal device 114 may implement the query 704 and then provide the IMD102 with instruction regarding whether the controller 204 should thenproceed with the remainder of these operations by beginning at the queryoperation 708 or at setting the non-pacing mode at as shown at state710. The controller 204 may then proceed with the remainder of theseoperations where the controller 204 then utilizes pre-exposure modeinformation without being required to uplink that information to theexternal device 114. However, where the external device 114 performs allof these operations or where the query 704 is based to some degree onpre-exposure mode programming, then the pre-exposure mode information isuplinked from the IMD 102 to the external device 114 and the externaldevice 114 downlinks the instruction for the proper exposure mode andrate.

It is also to be noted that as in the example of FIG. 4, at queryoperation 704, it is possible in some embodiments that no pre-exposuremode information has been needed because the type of device is entirelyhardware based and not affected by pre-exposure programming. In thatcase an IMD 102 that has the ability to automatically enter the exposuremode may be programmed by the manufacturer regarding which branch oflogic to use in FIG. 7 that stems from the query operation 704. In thatcase, the initial programming being loaded onto the IMD 102 from themanufacturer utilizes the device type information 702 to specify in theinitial programming which operation or state should be the firstoperation to be performed by the IMD 102 upon automatically enteringexposure mode, either query operation 708 or non-pacing mode 710. Inthat case, the IMD 102 need not perform the query operation 704 uponentering the exposure mode since that query 704 was already resolved atthe time of initial programming.

In any of these alternatives above, operational flow proceeds to query708 where the device type is such that pacing is available and proceedsto the state 710 otherwise. For device types leading to the queryoperation 708, the controller determines from pacing percentageinformation 706 stored by the IMD 102 whether the pacing percentage(considering atrial and/or ventricular) is greater than a threshold. Thepacing percentage information 706 may be based on pacing activity forsome pre-exposure period immediately preceding the exposure mode, suchas 15 minutes or for the entire day such as since midnight. For example,query 708 may determine whether the IMD 102 has been pacing the atriumor ventricle more than 5% of the pacing cycles for the 15 minutespreceding the exposure mode. It will be appreciated that other pacingpercentage thresholds may be also be applied but 5% is believed to beeffective for most patients. If the pacing percentage is lower than thethreshold, this indicates that it is appropriate for the patient to gowithout pacing for a short time such as for the duration of an MRI scan,and in that case the controller selects that there be no pacing viapacing mode OXO for the exposure mode as indicated at pacing state 710.

If the pacing percentage at the query operation 708 is greater than thepacing threshold, then the controller determines at the sequence ofquery operations 712 and 718 what the pre-exposure pacing mode has beenby referencing the pre-exposure mode pacing mode information 714. Thispre-exposure pacing mode, also referred to as the permanent pacing mode,is AXX/AXXR, VXX/VXXR, or DXX/DXXR. The controller sets the exposuremode to pace the same set of chambers as the pre-exposure mode, so wherethe pre-exposure pacing mode is AXX/AXXR, then in this embodiment thecontroller selects that there be atrial pacing for the exposure mode byusing asynchronous pacing mode AOO as indicated at pacing state 716.Where the pre-exposure pacing mode is VXX/VXXR, then in this embodimentthe controller selects that there be ventricular pacing for the exposuremode by using asynchronous pacing mode VOO as indicated at pacing state720. Where the pre-exposure pacing mode is DXX/DXXR, then in thisembodiment the controller performs an additional query operation 722 todetermine if there is an atrial fibrillation or mode switch related toan episode of atrial fibrillation that is in progress at the presenttime based on mode information 724. If so, then even if the permanentmode is dual pacing mode, the controller may select VOO mode at thepacing state 720 to avoid pacing the atrium and conserve energy as theatrial fibrillation may otherwise render the atrial pacing energysuperfluous. When there is no atrial fibrillation or mode switch inprogress, the controller selects that there be dual chamber pacing forthe exposure mode by using asynchronous pacing mode DOO as indicated atpacing state 726.

Once the pacing mode for the exposure mode has been set, the controllerthen proceeds to a query operation 730 to determine an appropriateexposure mode rate for the chosen pacing mode. The query operation 730determines whether a pre-exposure mode rate of cardiac activity asspecified in pre-exposure mode information 728 exceeds a rate threshold.The pre-exposure mode information 728 may be one of various differentpieces of information. For example, the pre-exposure mode information728 may specify an average ventricular rate taken over a 15 minuteperiod immediately preceding the exposure mode. This pre-exposure modeaverage rate may be an average intrinsic rate that has been sensed or itmay be an average paced rate.

It will be appreciated that the rate threshold applied at the queryoperation 730 may vary but it has been found that a fixed rate even aslow as 40 beats per minute is an effective example. If the threshold isnot exceeded by the pre-exposure mode rate, then the controller sets theasynchronous pacing rate to a pre-defined fixed value such as 50 beatsper minute as indicated at the pacing rate state 732. It will beappreciated that this pre-defined fixed value may vary, but 50 beats perminute are also known to be acceptable for many patients. If thethreshold is exceeded by the pre-exposure mode rate, then the controllersets the asynchronous pacing rate to a fixed value that is equal to thepre-exposure mode rate plus a fixed adjustment but with an upper limitat the pacing rate state 734. For example, the controller may set thatasynchronous pacing rate to the pre-exposure rate plus 10 additionalpaces per minute with an upper limit of 110 paces per minute. The fixedadjustment may vary but it has been found that an adjustment as low as10 additional paces per minute adequately avoids a competitive pacingcondition. Furthermore, the upper limit may vary but it has been foundthat an upper limit of 110 paces per minute is adequate. Therefore, inthis particular example, the pacing rate for the exposure mode will beset within the range of 50-110 paces per minute.

FIG. 8 shows another example 800 of logical operations that may be usedto determine the exposure pacing mode to be implanted based on devicetype, where the device type considerations are whether the device iscapable of multi-chamber pacing (i.e., Dual Chamber (D/C) or TripleChamber (T/C)). Initially, the type of IMD 102 is determined at a query804 that accesses the device type information 802. Similar to the priorexamples, where the controller performing these operations is theexternal device 114 being used to manually program the IMD 102 to enterthe exposure mode, the external device 114 may send a wireless requestfor information at the query 804 to obtain the device type 802 from theIMD 102. Where the controller 202 of the IMD 102 is the controllerperforming these operations either in response to a command from theexternal device 114 to enter the exposure mode or where the IMD 102 hasdetected the magnetic disturbance and has automatically entered theexposure mode, then the controller 202 accesses the locally storeddevice type information 802.

Regardless of which device performs these operations, query operation804 determines from the device type information 802 whether the IMD 102,and specifically the therapy device 208, is of the type that can providemulti-chamber pacing. Operational flow then proceeds to a queryoperation 812 when the IMD 102 does have multi-chamber pacing ability orotherwise proceeds to a query operation 806 when the device is onlysingle chamber pacing capable.

As with the prior examples, is also to be appreciated that an externaldevice 114 may implement the query 804 and then provide the IMD 102 withinstruction regarding whether the controller 204 should then proceedwith the remainder of these operations by beginning at the queryoperation 812 or the query operation 806. The controller 204 may thenproceed with the remainder of these operations where the controller 204then utilizes pre-exposure mode information without being required touplink that information to the external device 114. However, where theexternal device 114 performs all of these operations or where the query804 is based to some degree on pre-exposure mode programming, then thepre-exposure mode information is uplinked from the IMD 102 to theexternal device 114 and the external device 114 downlinks theinstruction for the proper exposure mode and rate.

It is also to be noted that as in the prior examples, at query operation804, it is possible in some embodiments that no pre-exposure modeinformation has been needed because the type of device is entirelyhardware based and not affected by pre-exposure programming. In thatcase an IMD 102 that has the ability to automatically enter the exposuremode may be programmed by the manufacturer regarding which branch oflogic to use in FIG. 8 that stems from the query operation 804. In thatcase, the initial programming being loaded onto the IMD 102 from themanufacturer utilizes the device type information 802 to specify in theinitial programming which operation or state should be the firstoperation to be performed by the IMD 102 upon automatically enteringexposure mode, either query operation 812 or query operation 806. Inthat case, the IMD 102 need not perform the query operation 804 uponentering the exposure mode since that query 804 was already resolved atthe time of initial programming.

In any of these alternatives above, operational flow proceeds to query812 where the device type is such that multi-chamber pacing is availableand proceeds to the query operation 806 otherwise. For device typesleading to the query operation 808, the controller determines frompacing percentage information 810 stored by the IMD 102 whether thepacing percentage of either of the multi-chambers (considering atrialand/or right ventricular pacing in this particular example of amulti-chamber device) is greater than a threshold. The pacing percentageinformation 810 may be based on pacing activity for some pre-exposureperiod immediately preceding the exposure mode, such as 15 minutes orfor the entire day such as since midnight. For example, query 812 maydetermine whether the IMD 102 has been pacing the atrium or rightventricle more than 5% of the pacing cycles for the 15 minutes precedingthe exposure mode. It will be appreciated that other pacing percentagethresholds may be also be applied but 5% is believed to be effective formost patients. If the pacing percentage for both chambers is lower thanthe threshold, this indicates that it is appropriate for the patient togo without pacing for a short time such as for the duration of an MRIscan, and in that case the controller selects that there be no pacingvia pacing mode OXO for the exposure mode as indicated at pacing state808.

For device types leading instead to the query operation 806, thecontroller determines from pacing percentage information stored by theIMD 102 for the single chamber pacing whether the pacing percentage ofthe single chamber is less than a threshold (i.e., does not meet orexceed the threshold). As in the prior examples, the pacing percentageinformation may be based on pacing activity for some pre-exposure periodimmediately preceding the exposure mode, such as 15 minutes or for theentire day such as since midnight. The device type may again be used inquery operation 806, where one threshold may be applied for devices withdefibrillation capability (e.g., 10%) while another threshold is appliedfor devices with only pacing capability (e.g., 5%) considering thatdevices with defibrillation capability and low pacing percentage areeven more likely to indicate a patient that is tolerant of a non-pacingmode for a relatively short duration such as the length of an Mill scan.It will be appreciated that other pacing percentage thresholds may bealso be applied but 10% and 5% are believed to be effective thresholdsfor most patients. If the pacing percentage is lower than the relevantthreshold, this indicates that it is appropriate for the patient to gowithout pacing during exposure mode, and in that case the controllerselects that there be no pacing via pacing mode OXO for the exposuremode as indicated at pacing state 808.

If the pacing percentage for the multi-chamber device at the queryoperation 812 is greater than the pacing threshold or if the pacingpercentage for the single chamber device at the query operation 806 isnot less than the device specific single chamber pacing threshold, thenthe controller determines the exposure mode via the sequence of queryoperations 814 and 820. The controller considers what the pre-exposurepacing mode has been by referencing the pre-exposure mode pacing modeinformation 816. This pre-exposure pacing mode, also referred to as thepermanent pacing mode, is AXX, VXX, or DXX. The controller sets theexposure mode to pace the same set of chambers as the pre-exposure mode,so where the pre-exposure pacing mode is AXX, then in this embodimentthe controller selects that there be atrial pacing for the exposure modeby using asynchronous pacing mode AOO as indicated at pacing state 818.Where the pre-exposure pacing mode is VXX, then in this embodiment thecontroller selects that there be ventricular pacing for the exposuremode by using asynchronous pacing mode VOO as indicated at pacing state822. Where the pre-exposure pacing mode is DXX, then in this embodimentthe controller selects that there be dual chamber pacing for theexposure mode by using asynchronous pacing mode DOO as indicated atpacing state 824.

Once the pacing mode for the exposure mode has been set, the controllerthen proceeds to a query operation 830 to determine an appropriateexposure mode rate for the chosen pacing mode. The query operation 830determines whether a pre-exposure mode rate of cardiac activity asspecified in either pre-exposure mode information 826 or alternatepre-exposure mode information 828 exceeds a rate threshold. Thepre-exposure mode information 826 may be one of various different piecesof information. For example, the pre-exposure mode information 828 mayspecify an average atrial and/or ventricular rate taken over a 15 minuteperiod immediately preceding the exposure mode. This pre-exposure modeaverage rate may be an average intrinsic rate that has been sensed or itmay be an average paced rate. The alternate pre-exposure modeinformation 828 may be a statistical distribution, such as a histogram,based set of information for atrial and/or ventricular cardiac activity,also either paced or intrinsic. This histogram or other statisticaldistribution information 828 may cover cardiac activity over the entirelife of the device or over a particular period of time. A particularpercentile from the histogram or other statistical distribution may bechosen as the representative value of pre-exposure mode rate. While thepercentile may vary from one embodiment to the next, it has been foundthat the 75^(th) percentile is an effective value.

It will be appreciated that the rate threshold applied at the queryoperation 830 may vary but it has been found that a moderate fixed ratelike 55 beats per minute is an effective example. If the threshold isnot exceeded by the pre-exposure mode rate, then the controller sets theasynchronous pacing rate to a pre-defined fixed value such as 70 pacesper minute as indicated at the pacing rate state 832. It will beappreciated that this pre-defined fixed value may vary, but 70 paces perminute are also known to be acceptable for many patients. If thethreshold is exceeded by the pre-exposure mode rate, then the controllersets the asynchronous pacing rate to a fixed value that is equal to thepre-exposure mode rate plus a fixed adjustment but with an upper limitat the pacing rate state 834. For example, the controller may set thatasynchronous pacing rate to the pre-exposure average rate plus 15additional paces per minute or the 75^(th) percentile rate plus 10additional paces per minute, with an upper limit of 110 paces per minutein either case. The fixed adjustment may vary but it has been found thatan adjustment as low as 10 additional paces per minute over the 75^(th)percentile or 15 paces per minute over the recent average adequatelyavoids a competitive pacing condition. Furthermore, the upper limit mayvary but it has been found that an upper limit of 110 paces per minuteis adequate. Therefore, in this particular example, the pacing rate forthe exposure mode will be set within the range of 70-110 paces perminute.

FIG. 9A shows another example 900 of logical operations that may be usedto determine the exposure pacing mode to be implanted based on devicetype. The example 900 is essentially the same as the example 800 shownin FIG. 8 except that when setting a pacing mode where appropriate,there is both a triggered mode and an asynchronous mode (also referredto as a noise pacing mode) that is selected by the controller.Initially, the type of IMD 102 is determined at a query 904 thataccesses the device type information 902. Query operation 904 determinesfrom the device type information 902 whether the IMD 102, andspecifically the therapy device 208, is of the type that can providemulti-chamber pacing. Operational flow then proceeds to a queryoperation 912 when the IMD 102 does have multi-chamber pacing ability orotherwise proceeds to a query operation 906 when the device is onlysingle chamber pacing capable.

As with the prior examples, is also to be appreciated that an externaldevice 114 may implement the query 904 and then provide the IMD 102 withinstruction regarding whether the controller 204 should then proceedwith the remainder of these operations by beginning at the queryoperation 912 or the query operation 906. The controller 204 may thenproceed with the remainder of these operations where the controller 204then utilizes pre-exposure mode information without being required touplink that information to the external device 114. However, where theexternal device 114 performs all of these operations or where the query904 is based to some degree on pre-exposure mode programming, then thepre-exposure mode information is uplinked from the IMD 102 to theexternal device 114 and the external device 114 downlinks theinstruction for the proper exposure mode and rate.

It is also to be noted that as in the prior examples, at query operation904, it is possible in some embodiments that no pre-exposure modeinformation has been needed because the type of device is entirelyhardware based and not affected by pre-exposure programming. In thatcase an IMD 102 that has the ability to automatically enter the exposuremode may be programmed by the manufacturer regarding which branch oflogic to use in FIG. 9A that stems from the query operation 904. In thatcase, the initial programming being loaded onto the IMD 102 from themanufacturer utilizes the device type information 902 to specify in theinitial programming which operation or state should be the firstoperation to be performed by the IMD 102 upon automatically enteringexposure mode, either query operation 912 or query operation 906. Inthat case, the IMD 102 need not perform the query operation 904 uponentering the exposure mode since that query 904 was already resolved atthe time of initial programming.

In any of these alternatives above, operational flow proceeds to query912 where the device type is such that multi-chamber pacing is availableand proceeds to the query operation 906 otherwise. For device typesleading to the query operation 912, the controller determines frompacing percentage information 910 stored by the IMD 102 whether thepacing percentage of either of the multi-chambers (considering atrialand/or right ventricular pacing in this particular example of amulti-chamber device) is greater than a threshold. The pacing percentageinformation 910 may be based on pacing activity for some pre-exposureperiod immediately preceding the exposure mode, such as 15 minutes orfor the entire day such as since midnight. For example, query 912 maydetermine whether the IMD 102 has been pacing the atrium or rightventricle more than 5% of the pacing cycles for the 15 minutes precedingthe exposure mode. It will be appreciated that other pacing percentagethresholds may be also be applied but 5% is believed to be effective formost patients. If the pacing percentage for both chambers is lower thanthe threshold, this indicates that it is appropriate for the patient togo without pacing for a short time such as for the duration of an MRIscan, and in that case the controller selects that there be no pacingvia pacing mode OXO for the exposure mode as indicated at pacing state908.

For device types leading instead to the query operation 906, thecontroller determines from pacing percentage information stored by theIMD 102 for the single chamber pacing whether the pacing percentage ofthe single chamber is less than a threshold (i.e., does not meet orexceed the threshold). As in the prior examples, the pacing percentageinformation may be based on pacing activity for some pre-exposure periodimmediately preceding the exposure mode, such as 15 minutes or for theentire day such as since midnight. The device type may again be used inquery operation 906, where one threshold may be applied for devices withdefibrillation capability (e.g., 10%) while another threshold is appliedfor devices with only pacing capability (e.g., 5%) considering thatdevices with defibrillation capability and low pacing percentage areeven more likely to indicate a patient that is tolerant of a non-pacingmode for a relatively short duration such as the length of an MRI scan.It will be appreciated that other pacing percentage thresholds may bealso be applied. If the pacing percentage is lower than the relevantthreshold, this indicates that it is appropriate for the patient to gowithout pacing during exposure mode, and in that case the controllerselects that there be no pacing via pacing mode OXO for the exposuremode as indicated at pacing state 908.

If the pacing percentage for the multi-chamber device at the queryoperation 912 is greater than the pacing threshold or if the pacingpercentage for the single chamber device at the query operation 906 isnot less than the device specific single chamber pacing threshold, thenthe controller determines the exposure mode via the sequence of queryoperations 914 and 920. The controller considers what the pre-exposurepacing mode has been by referencing the pre-exposure mode pacing modeinformation 916. This pre-exposure pacing mode, also referred to as thepermanent pacing mode, is AXX/AXXR, VXX/VXXR, or DXX/DXXR. Thecontroller sets the exposure mode to pace the same set of chambers asthe pre-exposure mode. In this embodiment, the controller selects anasynchronous or noise pacing mode as well as a triggered pacing modebased on the chamber(s) being paced in the pre-exposure mode. So wherethe pre-exposure pacing mode is AXX/AXXR, then in this embodiment thecontroller selects that there be atrial pacing for the exposure mode byselecting an asynchronous pacing mode AOO and a triggered pacing modeAAT as indicated at pacing state 918. Where the pre-exposure pacing modeis VXX/VXXR, then in this embodiment the controller selects that therebe ventricular pacing for the exposure mode by selecting an asynchronouspacing mode VOO and a triggered pacing mode VVT as indicated at pacingstate 922. Where the pre-exposure pacing mode is DXX/DXXR, then in thisembodiment the controller selects that there be dual chamber pacing forthe exposure mode by selecting an asynchronous pacing mode DOO and atriggered pacing mode DDT as indicated at pacing state 924.

Once the pacing mode for the exposure mode has been set, the controllerthen proceeds to a query operation 930 to determine an appropriateasynchronous exposure mode rate for the chosen pacing mode. The queryoperation 930 determines whether a pre-exposure mode rate of cardiacactivity as specified in either pre-exposure mode information 926 oralternate pre-exposure mode information 928 exceeds a rate threshold.The pre-exposure mode information 826 may be one of various differentpieces of information. For example, the pre-exposure mode information828 may specify an average atrial and/or ventricular rate taken over a15 minute period immediately preceding the exposure mode. Thispre-exposure mode average rate may be an average intrinsic rate that hasbeen sensed or it may be an average paced rate. The alternatepre-exposure mode information 828 may be a statistical distribution,such as a histogram, based set of information for atrial and/orventricular cardiac activity, also either paced or intrinsic. Thishistogram or other statistical distribution information 828 may covercardiac activity over the entire life of the device or over a particularperiod of time. A particular percentile from the histogram, such as the75^(th) percentile for instance, may be chosen as the representativevalue of pre-exposure mode rate. As noted in the prior example 800, thepercentile may vary from one embodiment to the next, but it has beenfound that the 75^(th) percentile is an effective value.

It will again be appreciated that the rate threshold applied at thequery operation 930 may vary but it has been found that a moderate fixedrate like 55 beats per minute is an effective example. If the thresholdis not exceeded by the pre-exposure mode rate, then the controller setsthe asynchronous pacing rate to a pre-defined fixed value such as 70paces per minute as indicated at the pacing rate state 932. It will beappreciated that this pre-defined fixed value may vary, but 70 paces perminute are also known to be acceptable for many patients. If thethreshold is exceeded by the pre-exposure mode rate, then the controllersets the asynchronous pacing rate to a fixed value that is equal to thepre-exposure mode rate plus a fixed adjustment but with an upper limitat the pacing rate state 934. As in the example 800 of FIG. 8, thecontroller may set that asynchronous pacing rate to the pre-exposureaverage rate plus 15 additional paces per minute or the 75^(th)percentile rate plus 10 additional paces per minute, with an upper limitof 110 paces per minute in either case. The fixed adjustment may vary ineither case as previously discussed. Furthermore, the upper limit mayvary but as previously stated it has been found that an upper limit of110 paces per minute is adequate. Therefore, in this particular example,the pacing rate for the exposure mode will be set within the range of70-110 paces per minute.

Because the exposure mode of this embodiment has a triggered mode and anasynchronous mode, additional logic is utilized to determine when to useone versus the other based on whether the intrinsic mode is likely to bephysiologic or artificial. One example 940 of this additional logic isshown in FIG. 9B. Here at a query operation 942, the controllerdetermines whether the sensed intrinsic rate is within an expectedrange. As discussed above in relation to the example of FIG. 5A, theexpected or normal range of the intrinsic rate may be determined basedon a range of percentiles from the pre-exposure histogram or otherstatistical distribution information, centered on the averagepre-exposure rate, or from a pre-defined range. As another example, theexpected range may be specified as anything equal to or lesser than thepreviously established asynchronous or noise pacing rate from FIG. 9A.As yet another example, the expected range may be based on the formula220-age of the patient to establish an upper limit on the expectedrange.

If the intrinsic rate is within the expected range, which suggests theintrinsic rate is physiologic rather than artificially produced by amagnetic disturbance, the controller then activates the triggered pacingmode, which has been previously established in FIG. 9A, at a pacingstate 946. If the intrinsic rate is outside of the expected range, thenthe controller activates the asynchronous pacing mode and correspondingasynchronous pacing rate, both of which have also been previouslyestablished in FIG. 9A, at a pacing state 944. This process of thecontroller determining whether the sensed intrinsic rate is in theexpected range and implementing either the triggered or asynchronousmode repeats throughout the duration of the exposure mode bycontinuously sensing the intrinsic rate and continuously determiningwhich mode of pacing to activate.

Another example 950 of the additional logic that may be used by thecontroller when choosing between triggered pacing mode and asynchronouspacing mode as the exposure mode of therapy is shown in FIG. 9C. In thisexample, the pacing mode previously established in FIG. 9A is for dualmode pacing. However, these logical operations account for thepossibility that the sensed intrinsic rate of one chamber to be pacedmay be in the expected range while the sensed intrinsic rate of theother chamber to be paced may not.

In this example, the controller detects whether the sensed intrinsicatrial rate is in the expected range at a query operation 952. Again,the expected or normal range for the atrial rate may be specified invarious ways such as in terms of a range of percentiles of the histogramof atrial rate data, a range centered on the pre-exposure atrial rateaverage, or a pre-defined range. As another example, the expected atrialrange may be specified as anything equal to or lesser than thepreviously established asynchronous or noise pacing rate from FIG. 9A.As yet another example, the expected atrial range may be based on theformula 220-age of the patient to establish an upper limit on theexpected range. If the atrial rate is expected, then the controllerdetermines whether the ventricular intrinsic rate is within the expectedrange at operation 960. The expected or normal range for the ventricularrate may also be specified in various ways such as in terms of a rangeof percentiles of the histogram of ventricular rate data, a rangecentered on the pre-exposure ventricular average, or a pre-definedrange. As another example, the expected ventricular range may bespecified as anything equal to or lesser than the previously establishedasynchronous or noise pacing rate from FIG. 9A. As yet another example,the expected ventricular range may be based on the formula 220-age ofthe patient to establish an upper limit on the expected range.

When the ventricular rate is also within the expected range, then thecontroller determines that the dual chamber triggered pacing mode isappropriate and then selects the dual chamber triggered mode DDT atpacing state 962. When the ventricular rate is not within the expectedrange, suggesting that it is not physiologic, even though the atrialrate is within the expected range, the dual chamber asynchronous mode ismost appropriate. Therefore, the controller then selects the dualchamber asynchronous mode DOO at the pacing mode state 956.

Returning to query operation 952, when the controller determines thatthe atrial rate is not within the expected range, the controller thendetermines whether the ventricular rate is within the expected range ata query operation 954. If the ventricular rate is also outside of theexpected range, then the dual chamber asynchronous mode is again mostappropriate. Therefore, the controller then selects the dual chamberasynchronous mode DOO at the pacing mode state 956. If the ventricularrate is within the expected range, then switching away from dual chamberpacing to allow for triggered ventricular pacing is most appropriate.Therefore, the controller then selects the triggered ventricular modeVVT at the pacing mode state 958. This process of the controllerdetermining whether the sensed intrinsic atrial and ventricular ratesare in the expected range and implementing the triggered or asynchronousdual chamber mode or the triggered ventricular mode repeats throughoutthe duration of the exposure mode by continuously sensing the intrinsicatrial and ventricular rates and continuously determining which mode ofpacing to activate.

FIG. 9D shows another example 941 of logical operations, similar to thatof FIG. 9B, except that in FIG. 9D electrogram (EGM) parameters capturedby the device 102 are analyzed at a query operation 970 to determinewhether the EGM parameters are physiologic. One example of the EGMparameters includes the morphology of the cardiac signal, which can becompared to a template to determine whether the morphology matches thetemplate within a certain degree of accuracy. Another example of the EGMparameters includes the frequency content of the cardiac signal, wherefrequency content outside of a physiologic frequency band is indicativeof an artificial source. If the EGM parameters are physiologic, then asensed rate is considered to be a reliable physiologic rate that can beused for triggered pacing mode at the pacing state 946. If the EGMparameters are not considered to be in a physiologic range, then asensed rate is considered to be an unreliable artificial rate thatshould not be used for triggered pacing mode and so asynchronous pacingis chosen at the pacing state 944.

FIG. 9E shows another example 951 of logical operations, similar to thatof FIG. 9C, except that in FIG. 9E EGM parameters captured by the device102 are analyzed to determine if the EGM parameters are within thephysiologic range. In particular, query 970 detects whether atrial EGMparameters are within the physiologic range. If query 970 detects thatthe atrial EGM parameters are physiologic, then operational flowproceeds to query operation 976 where it is determined whether theventricular EGM parameters are within the physiologic range. Where theventricular EGM parameters are physiologic, then DDT mode is chosen atthe DDT state 962 and where the ventricular EGM parameters are notphysiologic, then DOO mode is chosen at the DOO state 956. If query 970detects that the atrial EGM parameters are not physiologic, thenoperational flow proceeds to query operation 972 where it is determinedwhether the ventricular EGM parameters are within the physiologic range.Where the ventricular EGM parameters are physiologic, then VVT mode ischosen at the VVT state 958 and where the ventricular EGM parameters arenot physiologic, then DOO mode is chosen at the DOO state 956. Someexamples of the atrial and ventricular EGM parameters may include thosediscussed above with reference to FIG. 9D, such as cardiac signalmorphology and frequency content.

FIG. 10 shows another example 1000 of logical operations that acontroller may employ to ultimately select the pacing mode and rate forthe exposure mode of therapy. Initially, the type of IMD 102 isdetermined at a query 1004 that accesses the device type information1002. For instance, where the controller performing these operations isthe external device 114 being used to manually program the IMD 102 toenter the exposure mode, the external device 114 may send a wirelessrequest for information at the query 1004 to obtain the device type 1002from the IMD 102. Where the controller 202 of the IMD 102 is thecontroller performing these operations either in response to a commandfrom the external device 114 to enter the exposure mode or where the IMD102 has detected the magnetic disturbance and has automatically enteredthe exposure mode, then the controller 202 accesses the locally storeddevice type information 1002.

Regardless of which device performs these operations, query operation1004 determines from the device type information 1002 whether the IMD102, and specifically the therapy device 208, is of the type that canprovide defibrillation. For instance, query operation 1004 mayspecifically check for an ICD or a CRT device with defibrillation.Operational flow then proceeds to a query operation 1006 when the IMD102 does have defibrillation ability or proceeds to query operation 1012when the IMD 102 does not have defibrillation ability.

It is also to be appreciated that an external device 114 may implementthe query 1004 and then provide the IMD 102 with instructions regardingwhether the controller 204 should then proceed with the remainder ofthese operations by beginning at the query operation 1006 or at thequery operation 1012. The controller 204 may then proceed with theremainder of these operations where the controller 204 then utilizespre-exposure mode information without being required to uplink thatinformation to the external device 114. However, where the externaldevice 114 performs all of these operations or where the query 1004 isbased to some degree on pre-exposure mode programming (e.g., CRT-D withdefibrillation deactivated to behave as a CRT-P), then the pre-exposuremode information is uplinked from the IMD 102 to the external device 114and the external device 114 downlinks the instruction for the properexposure mode and rate.

It is also to be noted that at query operation 1004, it is possible insome embodiments that no pre-exposure mode information has been neededbecause the type of device is entirely hardware based and not affectedby pre-exposure programming. In that case an IMD 102 that has theability to automatically enter the exposure mode may be programmed bythe manufacturer regarding which branch of logic to use in FIG. 10 thatstems from the query operation 1004. In that case, the initialprogramming being loaded onto the IMD 102 from the manufacturer utilizesthe device type information 1002 to specify in the initial programmingwhich query operation should be the first operation to be performed bythe IMD 102 upon automatically entering exposure mode, either queryoperation 1006 or query operation 1012. In that case, the IMD 102 neednot perform the query operation 1004 upon entering the exposure modesince that query 1004 was already resolved at the time of initialprogramming.

In any of these alternatives above, operational flow proceeds to query1006 where the device type is such that defibrillation is available andproceeds to the query operation 1012 otherwise. For device types leadingto the query operation 1006

For device types and configurations leading to the query operation 1008,the controller determines from the ventricular pacing percentage (VP %)information stored by the IMD 102 whether the ventricular pacingpercentage is less than a relatively low threshold, such as less than1%. The ventricular pacing percentage information may be based on pacingactivity for some pre-exposure period immediately preceding the exposuremode, such as 15 minutes or for the entire day such as since midnight.For example, query 1006 may determine whether the IMD 102 has beenpacing the ventricle less than 1% of the pacing cycles for the 15minutes preceding the exposure mode. As has been previously discussedherein, it will be appreciated that various pacing percentage thresholdsmay also be applied but 1% is believed to be an effective amount formost patients when applying this example of logical operations. If thepacing percentage is lower than the threshold, this indicates that it isappropriate for the patient to go without pacing for a short time suchas for the duration of an MRI scan, and in that case the controllerselects that there be no pacing via pacing mode OXO for the exposuremode as indicated at pacing state 1018. If the pacing percentage at thequery operation 1008 is greater than the pacing threshold, then theoperational flow proceeds directly to the query operation 1012.

At the sequence of query operations 1012, 1014, and 1016, the controllerthen determines what the pre-exposure pacing mode has been byreferencing the pre-exposure mode pacing mode information 1010. Aspreviously discussed, this pre-exposure pacing mode, also referred to asthe permanent pacing mode, is AXX/AXXR, VXX/VXXR, or DXX/DXXR but mayalso be a mode where pacing is not being provided but sensing may beoccurring. Query 1012 determines if the permanent pacing mode is suchthat no pacing is occurring and in that case sets the pacing mode to OXOat the operation 1018. The controller sets the exposure mode to pace thesame set of chambers as the pre-exposure mode, so where the pre-exposurepacing mode is AXX/AXXR, then in this embodiment the controller selectsthat there be atrial pacing for the exposure mode by using asynchronouspacing mode AOO as indicated at pacing state 1020. Where thepre-exposure pacing mode is VXX/VXXR, then in this embodiment thecontroller selects that there be ventricular pacing for the exposuremode by using asynchronous pacing mode VOO as indicated at pacing state1022. Where the pre-exposure pacing mode is DXX/DXXR, then in thisembodiment the controller selects that there be dual chamber pacing forthe exposure mode by using asynchronous pacing mode DOO as indicated atpacing state 1024.

Once the pacing mode for the exposure mode has been set, the controllerthen proceeds to a query operation 1026 to begin the determination of anappropriate exposure mode pacing rate for the chosen pacing mode. Thequery operation 1026 determines if the pacing mode is set to anon-pacing mode. If so, then the non-pacing mode with no pacing rate ismaintained at the pacing mode state 1028. If a mode that does pace hasbeen set then a subsequent query operation is performed depending uponthe circumstances of entering the MRI mode. If the implantable medicaldevice is being programmed into the MRI mode by the external instrument,then query operation 1034 may be performed, either by the implantabledevice upon being triggered by the external instrument or by theexternal instrument itself. At query operation 1034, it is determinedwhether a pre-exposure mode rate of cardiac activity as specified inpre-exposure mode information established by rate information 1030 andcalculation operation 1032 exceeds a rate threshold.

The pre-exposure mode information may be one of various different piecesof information. For example, the pre-exposure mode information mayspecify a median ventricular rate taken over a 15 minute periodimmediately preceding the exposure mode that is calculated by thecalculation operation 1032 from the rate information 1030. Thispre-exposure mode rate may be a median intrinsic rate that has beensensed or it may be a median paced rate. It will be appreciated thatother mathematical calculations besides a median calculation may beutilized, such as calculating the mean intrinsic rate or the mean pacedrate.

At the query operation 1034, where the MRI mode is being triggered bythe external device, it is determined whether the calculated heart rateexceeds a first threshold that is relatively high, such as 110 beats perminute. If so, then the MRI scan should not be performed without gettingan additional review and programming from the responsible cardiacclinician. Where the query operation 1034 is being performed by theexternal instrument, that instrument then signals to the clinician thatthe MRI scan should not be allowed without a review at an operation1036. Where the query operation 1034 is being performed by theimplantable medical device, the implantable medical device communicateswith the external instrument to signal that the MRI should not beperformed without a review, and then the external instrument signals thesame to the clinician at the operation 1036.

Where the calculated heart rate is found to be below the first thresholdat the query operation 1034, a query operation 1038 then determineswhether the heart rate exceeds a second threshold that is moderate, suchas 75 beats per minute. If the calculated rate is under the secondthreshold, then the pacing rate is set to the calculated heart rate plusa moderate upward adjustment amount, such as 15 beats per minute, toestablish a moderate fixed rate. Where the calculated heart rate isfound to be above the second threshold, then operational flow proceedsto a query operation 1044.

At the query operation 1044, it is determined whether the calculatedheart rate exceeds a third threshold and whether the VP %, provided asinformation 1042 which matches the information used at the queryoperation 1008 discussed above, exceeds a relatively high VP %threshold, such as 97%. The third threshold is between the moderatesecond threshold and the relative high first threshold, such as 90 beatsper minute. If the calculated heart rate does not exceed the thirdthreshold and/or the VP % does not exceed the VP % threshold, then thepacing rate is set to the calculated heart rate plus an upwardadjustment, typically smaller than the adjustment at operation 1040,such as 10 beats per minute at an operation 1046. Also, at operation1046, the rate is capped at with an upper limit, such as 120 beats perminute.

If both the third threshold is exceeded by the calculated heart rate andthe VP % exceeds the VP % threshold, then operation flow proceeds to aquery 1050. Here, it is determined whether the device type as specifiedin information 1048, which is the same as information 1002 discussedabove, is an implantable pulse generator or an implantabledefibrillator. If not, meaning the device is likely a cardiacresynchronization therapy device with or without defibrillation, thenoperational flow proceeds back to operation 1046 where the rate asdiscussed above for operation 1046 is applied. If the device is animplantable pulse generator or an implantable defibrillator, thenoperational flow proceeds to operation 1052. Here, the pacing rate isset to a moderate level such as 85 beats per minute. Note that whenoperational flow reaches operation 1052, the pacing mode is VOO.

Additional information related to the pacing mode includes parametersthat may remain fixed. A maximum pacing atrial to ventricular pacing(PAV) limit may be set to a fixed value such as 110 ms to minimize therisk of pacing on a T-wave. If the permanent PAV is already less thanthis value, then PAV interval may not be extended. Right Ventricular(RV) pacing amplitude and width and Right Atrial (RA) pacing amplitudeand width may be set to a fixed value to assure a safety pacing marginand pacing support during an MRI scan. LV amplitude may be maintained atan existing programmed setting for amplitude and width because setting ahigher amplitude may result in phrenic stimulations that hinder MRIimaging.

While embodiments have been particularly shown and described, it will beunderstood by those skilled in the art that various other changes in theform and details may be made therein without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of controlling an exposure mode oftherapy of an implantable medical device, comprising: determining adevice type of the implantable medical device by accessing device typeinformation stored by the implantable medical device; upon detecting aneed to switch to the exposure mode, selecting the exposure mode oftherapy of the implantable medical device based on the device type and apre-exposure mode of therapy; and after detecting the need to switch tothe exposure mode, implementing the exposure mode of therapy at theimplantable medical device by: performing a first sensing of a heartrate using at least one sensing setting; when the first sensing of heartrate indicates at least a first amount of noise is present, implementingan asynchronous pacing mode and then making adjustments to the at leastone sensing setting prior to a next sensing of the heart rate that issubsequent to the first sensing and that uses the at least one sensingsetting that has been adjusted; and when the first sensing of heart rateindicates that at least the first amount of noise is not present, thenimplementing a triggered pacing mode.
 2. The method of claim 1, furthercomprising: incrementing a revision counter upon making adjustments tothe at least one sensing setting; and subsequent to the next sensing ofthe heart rate, determining whether the revision counter exceeds arevision counter threshold so that when the revision counter does notexceed the revision counter threshold then making further adjustments tothe at least one sensing setting and when the revision counter doesexceed the revision counter threshold then not making furtheradjustments to the at least one sensing setting.
 3. The method of claim1, wherein determining the type of implantable medical device comprisesdetermining whether the implantable medical device is a type capable ofproviding a defibrillation therapy and when the implantable medicaldevice is not a type capable of providing defibrillation therapy, thenselecting the exposure mode based on device type and the pre-exposuremode comprises implementing a pacing mode that paces a same set ofchambers of the heart as the pre-exposure mode.
 4. The method of claim2, wherein when it is determined that the implantable medical device iscapable of providing defibrillation therapy, then selecting the exposuremode based on device type and the pre-exposure mode comprisesdetermining whether a pacing percentage of the device during a period oftime prior to implementing the exposure mode exceeds a pacing percentagethreshold and when the pacing percentage threshold is not exceeded thenselecting a non-pacing mode as the exposure mode.
 5. The method of claim4, wherein when the pacing percentage threshold is exceeded by thepacing percentage, then determining whether the pre-exposure mode of theimplantable medical device provides cardiac resynchronization therapy bypacing only a left ventricle of the heart and when the pre-exposure modedoes only pace the left ventricle, then selecting the exposure modebased on device type and the pre-exposure mode comprises selecting anon-pacing mode as the exposure mode.
 6. The method of claim 4, whereinwhen the pacing percentage threshold is exceeded by the pacingpercentage, then selecting the exposure mode based on device type andthe pre-exposure mode comprises implementing a pacing mode that paces asame set of chambers of the heart as the pre-exposure mode.
 7. Themethod of claim 2, wherein when it is determined that the implantablemedical device is capable of providing defibrillation therapy, thendetermining whether the pre-exposure mode of the implantable medicaldevice provides cardiac resynchronization therapy by pacing only a leftventricle of the heart and when the pre-exposure mode does only pace theleft ventricle, then selecting the exposure mode based on device typeand the pre-exposure mode comprises selecting a non-pacing mode as theexposure mode.
 8. The method of claim 7, wherein when it is determinedthat the pre-exposure mode does not provide cardiac resynchronizationtherapy by only pacing the left ventricle, then selecting the exposuremode based on device type and the pre-exposure mode comprisesimplementing a pacing mode that paces a same set of chambers of theheart as the pre-exposure mode.
 9. The method of claim 1, wherein priorto implementing the exposure mode of therapy there are pre-exposurerates of cardiac activity and wherein the exposure mode of therapyincludes providing pacing from the implantable medical device, themethod further comprising: upon detecting a need to switch to theexposure mode, selecting an exposure mode asynchronous pacing rate basedon the pre-exposure rates where the exposure mode asynchronous pacingrate is used when in the asynchronous pacing mode; and implementing theexposure mode asynchronous pacing rate at the implantable medical devicewhile implementing the exposure mode of therapy when the asynchronouspacing mode is being implemented.
 10. The method of claim 9, whereinselecting the exposure mode pacing rate based on the pre-exposure ratescomprises determining whether an average rate of cardiac activityoccurring during a period prior to implementing the exposure modeexceeds a rate threshold, when the average rate does not exceed thethreshold then pacing at a pre-defined fixed rate as the exposure modepacing rate and when the average does exceed the threshold then pacingat a rate equal to the average rate plus a fixed adjustment up to apre-defined upper limit.
 11. The method of claim 10, wherein thepre-exposure rates of cardiac activity are intrinsic rates.
 12. Themethod of claim 11, wherein the pre-exposure rates of cardiac activityare paced rates.
 13. An implantable medical system comprising: a devicefor providing electrical sensing and pacing; and a controller that is incontrol of the device to provide an exposure mode of therapy by thedevice and that is configured to determine a device type, detect a needto switch to the exposure mode, select the exposure mode of therapy ofthe device based on the device type and a pre-exposure mode of therapy,and after detecting the need to switch to the exposure mode implementthe exposure mode of therapy at the device by: performing a firstsensing of a heart rate using at least one sensing setting; when thefirst sensing of heart rate indicates at least a first amount of noiseis present, implementing an asynchronous pacing mode and then makingadjustments to the at least one sensing setting prior to a next sensingof the heart rate that is subsequent to the first sensing and that usesthe at least one sensing setting that has been adjusted; and when thefirst sensing of heart rate indicates that at least the first amount ofnoise is not present, then implementing a triggered pacing mode.
 14. Amethod of controlling an exposure mode of therapy of an implantablemedical device, comprising: determining a device type of the implantablemedical device; upon detecting a need to switch to the exposure mode,selecting the exposure mode of therapy of the implantable medical devicebased on the device type and a pre-exposure mode of therapy;implementing the exposure mode of therapy at the implantable medicaldevice by: performing a first sensing of a heart rate using at least onesensing setting; when the first sensing of heart rate indicates at leasta first amount of noise is present, implementing an asynchronous pacingmode and then making adjustments to the at least one sensing settingprior to a next sensing of the heart rate that is subsequent to thefirst sensing and that uses the at least one sensing setting that hasbeen adjusted; and when the first sensing of heart rate indicates thatat least the first amount of noise is not present, then implementing atriggered pacing mode; incrementing a revision counter upon makingadjustments to the at least one sensing setting; and subsequent to thenext sensing of the heart rate, determining whether the revision counterexceeds a revision counter threshold so that when the revision counterdoes not exceed the revision counter threshold then making furtheradjustments to the at least one sensing setting and when the revisioncounter does exceed the revision counter threshold then not makingfurther adjustments to the at least one sensing setting.
 15. A method ofcontrolling an exposure mode of therapy of an implantable medicaldevice, comprising: determining a device type of the implantable medicaldevice; upon detecting a need to switch to the exposure mode, selectingthe exposure mode of therapy of the implantable medical device based onthe device type and a pre-exposure mode of therapy; and implementing theexposure mode of therapy at the implantable medical device by:performing a first sensing of a heart rate using at least one sensingsetting; when the first sensing of heart rate indicates at least a firstamount of noise is present, implementing an asynchronous pacing mode andthen making adjustments to the at least one sensing setting prior to anext sensing of the heart rate that is subsequent to the first sensingand that uses the at least one sensing setting that has been adjusted;and when the first sensing of heart rate indicates that at least thefirst amount of noise is not present, then implementing a triggeredpacing mode; wherein determining the type of implantable medical devicecomprises determining whether the implantable medical device is a typecapable of providing a defibrillation therapy and when the implantablemedical device is not a type capable of providing defibrillationtherapy, then selecting the exposure mode based on device type and thepre-exposure mode comprises implementing a pacing mode that paces a sameset of chambers of the heart as the pre-exposure mode.
 16. A method ofcontrolling an exposure mode of therapy of an implantable medicaldevice, comprising: determining a device type of the implantable medicaldevice; upon detecting a need to switch to the exposure mode, selectingthe exposure mode of therapy of the implantable medical device based onthe device type and a pre-exposure mode of therapy; implementing theexposure mode of therapy at the implantable medical device by:performing a first sensing of a heart rate using at least one sensingsetting; when the first sensing of heart rate indicates at least a firstamount of noise is present, implementing an asynchronous pacing mode andthen making adjustments to the at least one sensing setting prior to anext sensing of the heart rate that is subsequent to the first sensingand that uses the at least one sensing setting that has been adjusted;and when the first sensing of heart rate indicates that at least thefirst amount of noise is not present, then implementing a triggeredpacing mode; wherein prior to implementing the exposure mode of therapythere are pre-exposure rates of cardiac activity and wherein theexposure mode of therapy includes providing pacing from the implantablemedical device, the method further comprising: upon detecting a need toswitch to the exposure mode, selecting an exposure mode asynchronouspacing rate based on the pre-exposure rates where the exposure modeasynchronous pacing rate is used when in the asynchronous pacing mode;and implementing the exposure mode asynchronous pacing rate at theimplantable medical device while implementing the exposure mode oftherapy when the asynchronous pacing mode is being implemented.